The quality of coffee prepared on an "espresso" coffee maker is dependent-on a variety of factors, which are tied both to the coffee used, namely its blend, quality and grinding grade, and the coffee maker itself, specifically the machine processing rate, operating pressure and water temperature. As may be appreciated, the choice of the machine-dependent parameters will be influenced by the coffee-dependent ones; it is for the designer to set these parameters and design the machine to keep such parameters continual as far as possible, irrespective of the operating conditions encountered, which may vary a great deal.
In particular, the temperature of the dispensation water flowed through the percolator loaded with a charge of ground coffee is recognized to be critical. Also well-recognized is that that temperature is quite difficult to control because it is readily affected by the machine own conditions of operation.
To provide ideal dispensation conditions, the machine may be equipped with indicator gauges to tell the operator whether the time is right for coffee dispensation or additional time should be allowed in order for the optimum temperature to be attained. This approach would only suit those applications where operators of adequate skill can be detached to supervise the coffee-making operation and continuously monitor the machine, and especially where the downtime involved in waiting for the ideal conditions to set in would not be objectionable.
But since such conditions are hardly met in practice, it remains for the machine itself to maintain best heating conditions for coffee making.
Accordingly, certain water supply systems have been developed which, additionally to supplying the dispenser units, provide for heat exchanges to occur within the machine between areas at different temperatures liable to swing in opposite directions during operation of the machine, thereby compensating to some extent for local temperature variations.
Such water supply systems are calibrated, as by the inclusion of waterflow throttling or metering passageways, for normal operating conditions; their effectiveness being lost under extreme conditions, such as during the machine turn-on transient or where large amounts of water are to be delivered in close succession (e.g. to dispense weaker coffee, as may typically be the case in Northern Countries).
A coffee maker is disclosed by DE-A-2747990, where a water supply system is provided including delivery and return pipes between a boiler and dispensing units; circulation of water in the water supply system helps in keeping dispensing units warm.
Another coffee maker is known from CH-A-365194, where in the dispensing units warm water and cold water are mixed under control of an automatic valve, driven by the temperature of the warm water.